1. Field of the Invention
The present invention relates to a polishing slurry, particularly, a polishing slurry used in a process of forming wiring of a semiconductor device and to a polishing method using the polishing slurry.
2. Description of the Related Art
New technologies for fine processing have been recently developed along with the development of highly integrated and high performance semiconductor integrated circuits (LSIs). A Chemical mechanical polishing method (hereinafter referred to as CMP) is among these technologies and is techniques which are being frequently utilized to flatten an interlayer dielectric film, to form a metal plug and to form embedded wiring in a process of producing LSIs and, particularly, a process of forming multi-layer wiring. These techniques are disclosed in, for example, U.S. Pat. No. 4,944,836.
In recent years, an attempt has been made to utilize copper alloys as wiring materials to develop high performance LSIs. However, these copper alloys have difficulties in fine processing carried out by a dry etching method which is frequently used to form conventional aluminum alloy wiring. For this, a so-called damascene method is primarily adopted in which a copper alloy thin film is deposited on a dielectric film in which a groove has been formed in advance and embedded in this groove and the copper alloy thin film on the dielectric film except for the groove portion is removed by CMP to form embedded wiring. These technologies are disclosed in, for example, the publication of Japanese Patent Application Laid-Open (JP-A) No. 2-278822.
In a general method used for CMP of a metal, a polishing cloth (pad) is applied to a circular polishing table (platen), the surface of the polishing cloth is moistened with a polishing slurry, the surface of a base body on which surface a metal film is formed is pressed against the polishing cloth and the polishing table is rotated in the condition that specified pressure (polishing pressure or polishing load) is applied to the backside of the base body to remove the metal film of the convex parts through mechanical contact with the convex parts by the aid of the polishing slurry.
The polishing slurry used in CMP usually comprises an oxidant and abrasive, to which a metal oxide dissolver and a metal inhibitor are further added according to the need. It is considered to be a fundamental mechanism that first, the surface of the metal film is oxidized by oxidation and the resulting oxide layer is shaven off by the solid abrasive. The oxide layer on the metal surface in the concave parts is not almost in contact with the polishing cloth, so that the solid abrasive has no shaving effect on that oxide layer in the concave parts. Therefore, along with the progress of CMP, the metal layer on the convex parts is removed and the surface of the base body is flattened. The details of this mechanism are disclosed in Journal of Electrochemical Society, Vol. 138, No. 11 (issued in 1991), pp 3460-3464.
It is regarded as effective to add a metal oxide dissolver as a method of raising polishing rate by CMP. The interpretation of such a result may be that when metal oxide grains shaven off by the solid abrasive is dissolved in the polishing slurry, the shaving effect of the solid abrasive is increased. However, if the oxide layer on the surface of the metal film in the concave parts is also etched and the surface of the metal film is thereby exposed, the surface of the metal oxide film is more oxidized by the oxidant. The repetition of this process leads to the progress of etching of the metal film in the concave parts and there is therefore a fear that the flattening effect is impaired. In order to wipe away this fear, a metal inhibitor is added. In order to maintain flattening characteristics, it is important to balance the effect of the metal oxide dissolver with that of the metal inhibitor and it is desirable that the oxide layer on the surface of the metal film in the convex parts is not much etched but grains of the oxide layer shaven off be dissolved effectively, resulting in a high polishing rate in CMP.
An acid as the metal oxide dissolver and the metal inhibitor are added in this manner to add the effect of a chemical reaction, whereby not only the polishing rate in CMP is improved but also such an effect is obtained that damages to the surface of the metal layer which surface is polished chemically and mechanically are reduced.
However, the formation of wiring by conventional CMP poses the problems, for example, (1) the occurrence of a phenomenon (dishing) that the center part of the surface of the embedded metal wiring is isotropically etched to form a dish-like cave and the occurrence of a phenomenon (erosion or thinning) that the dielectric film is also etched in a part having high wiring density and the thickness of the metal wiring is reduced, (2) the occurrence of polishing scratches, (3) a complicated washing process used to remove a polishing residue left on the surface of the base body after the base body is polished, (4) increased cost caused by waste fluid treatment and (5) corrosion of metals.
In order to suppress dishing and the corrosion of a copper alloy during polishing to thereby form highly reliable LSI wiring, a method using a polishing slurry containing a metal oxide dissolver including an aminoacetic acid such as glycine or sulfuric acid amide and BTA (benzotriazole) is proposed. These technologies are described in, for example, the publication of JP-A No. 8-83780.
In the formation of an embedded metal such as the formation of damascene wiring of copper or a copper alloy and the formation of plug wiring of tungsten, such an erosion phenomenon occurs that the wiring including the interlayer dielectric film is made thin when the rate of the polishing of a silicone dioxide film which is the interlayer dielectric film formed on a portion except for the embedded portion is also high. As a result, an increase in wiring resistance and pattern density causes the dispersion of resistance and it is therefore demanded of the polishing slurry to have the characteristics that the rate of the polishing of the silicone dioxide film is sufficiently lower than that of the metal film to be polished. For this, a method is proposed in which the pH of a polishing slurry is made larger than (pKa-0.5) to limit the speed of the polishing of silicon dioxide by anions produced by the dissociation of an acid. These technologies are described in, for example, the publication of Japanese Patent No. 2819196.
In the meantime, a barrier layer constituted of tantalum, titanium, tungsten or tantalum nitride, titanium nitride, tungsten nitride and a tantalum alloy, titanium alloy, tungsten alloy or other tantalum compounds, titanium compounds and tungsten compounds is formed as a layer disposed under the copper or copper alloy wiring to prevent copper from diffusing into the interlayer dielectric film. It is therefore necessary to remove the exposed barrier layer except for that of the wiring part used to embed copper or a copper alloy by CMP. However, these barrier layer conductive film has higher hardness than copper or a copper alloy and a combination of polishing materials used for copper or a copper alloy therefore fails in obtaining sufficient CMP rate, giving rise to the problem that dishing of copper or a copper alloy is caused, which decreases the thickness of the wiring during the removal of the barrier layer by CMP. Therefore, studies are being made as to a two-stage polishing method involving a first step of polishing copper or a copper alloy and a second step of polishing the barrier layer conductor.
The polishing slurry used to polish copper or a copper alloy can succeed in obtaining high polishing rate by adding a metal oxide dissolver to adjust the pH to the acidic side. In the acidic range, however, copper or copper alloy wiring is easily corroded. For this, it is necessary to increase the concentration of a corrosion inhibitor in the polishing slurry, to decrease the concentration of the metal oxide dissolver in the polishing slurry or to use a less corrosive acid having a high pKa.
In recent LSI fields, studies have been made as to technologies using an interlayer dielectric film having a small dielectric constant to decrease the signal delay time of metal wiring. However, because these interlayer films and laminate film structure using these interlayer films have small mechanical strength, wiring inferiors are easily caused by, for example, peeling at the boundary between laminate films. This is the reason why it has been desired to reduce friction in a polishing process.
As regards the polishing slurry used to polish copper or a copper alloy, if the concentration of the corrosion inhibitor is increased when an acid having a low pKa is used and the concentration of the metal oxide dissolver is increased to obtain a high polishing rate, the coefficient of polishing friction (coefficient of dynamic friction) during polishing tends to be increased, giving rise to problems concerning, for example, peeling and wire breaking of metal wiring and its laminate film structure. Also, a rise in temperature caused by friction leads to an increase in the chemical etching effect of the polishing slurry, causing the metal wiring to be etched, posing problems, for example, large dishing.
On the other hand, when an acid which has a high pKa and is hence less corrosive is used and the concentration of the metal oxide dissolver is dropped, there is the problem that only insufficient polishing rate is obtained. Also, when the pH of the polishing slurry is increased, only insufficient polishing rate is obtained.
In view of this situation, it is an object of the present invention to provide a polishing slurry which has a high metal-polishing rate, reduces etching rate and friction during polishing, and hence results high productivity and reduces dishing and erosion, and to provide a polishing method using the polishing slurry. The present invention can provide a highly reliable semiconductor device which is minimized, thin-filmed and is superior in dimensional accuracy and electric characteristics.